Needle valve mechanisms are often employed in a shaft of a fluid dispensing device to control the flow of the fluid. For example, conventional paint spray guns generally employ a needle valve stem to control the flow of the paint. In such devices, a sealing means is present about the needle valve stem to prevent the sprayable fluid from leaking out of the spray gun shaft, e.g., out of or into the body of the gun, when the needle valve stem is moved back and forth. Commercially available spray gun needle valve stem seals conventionally comprise a packing of string and graphite or teflon felt. Generally, the packing materials are mechanically compressed around the stem by means of a packing gland to form a seal. As the needle valve is moved back and forth in order to start and stop the paint flow, however, paint material eventually works its way into the interface between the packing and the needle valve stem which results in the binding of the needle. Even when the spray gun is frequently purged, the purge solvent does not effectively clean the needle in the area of the packing. The paint can also work its way into the porous packing and, particularly when the chemically reactive materials are two-component urethane paints which cure readily, deteriorate and harden the packing materials which then provide a less than effective seal.
As discussed in U.S. Pat. No. 4,560,109, conventional packing materials of this type wear down as the needle valve stem is repeatedly reciprocated and a packing gland is used in an attempt to tighten the packing and maintain a tight seal. This patent discloses that in prior art sealing means of this type, however, a tight seal cannot be obtained. The patent invention attempts to overcome this problem by providing a sealing device for a needle valve stem which comprises an annular main packing having a concave portion formed therein and an auxiliary packing having rubber elasticity fitted in the concave portion. According to the patent, as the main packing is worn away, a packing gland is tightened against the main packing which deforms the auxiliary packing, this auxiliary packing then uniformly presses the main packing against the circumference of the needle valve stem to maintain a seal. However, such a sealing device requires somewhat complex fabrication of the main and auxiliary packing. Additionally, the design of the main packing provides for a relatively high contact area between the needle valve stem and the main packing, and as a consequence, offers potential for undesirably high friction between the stem and main packing.
An object of the present invention is to provide a sealing means capable of providing a snug (fluid tight) seal for a cylindrical sliding portion of a needle valve stem used in a device for dispensing a fluid, i.e., to keep the fluid from leaking out rearwardly past the stem of the device. Another object of the present invention is provide a sealing means which has a relatively small surface area in contact with the needle valve stem and, as a result, very low friction between the seal and the needle valve stem. Another object of the present invention is to provide a seal which is not complex in design, is easily flushable, and which has improved efficiency at high paint pressures. It is yet another object of this invention to provide a sealing means which maintains a fluid tight seal on the needle valve stem even if a portion of the seal is worn off by the sliding action of the needle valve stem against the seal. Such objects are attained by the present invention which overcomes the deficiencies of prior art needle valves stem seals as discussed above.